1. Field of the Invention
The present invention relates to a respiratory measurement system. The primary components of the system include a respiratory air flow sensor, a microprocessor based module, lumen tubing for connecting the respiratory air flow sensor to the module, a connector for connecting the lumen tubing to the module, a mechanism for optionally purging the system, and a mechanism for optionally measuring content of a particular respiratory gas. This application specifically concerns an apparatus for analyzing the content of a specified respiratory gas, such as carbon dioxide (CO2) and nitrous oxide (N2O), using infrared spectroscopy.
2. Background of the Related Art
A patient receiving anesthesia or in intensive care, for example, needs to have his or her inhalations and exhalations continuously monitored. Respiratory mechanics refers to the monitoring of the physical parameters of a mechanically ventilated patient""s airway. The parameters include airway flow and pressure. Various measuring devices are used to measure the air flow rate. For some patients the content of particular respiratory gases flowing from or to the lungs must also be analyzed.
For measuring the air flow rate, it has been well known to use a tubular device which measures the pressure differential across a portion of the tube. An example of such a device is described in U.S. Pat. Nos. 5,535,633 and 5,379,650 (referred to hereafter collectively as xe2x80x9cthe ""633 devicexe2x80x9d). The ""633 device depends on the creation of a direct impedance to the axial gas flow through the tube in order to obtain the pressure differential from which the air flow rate can be derived by the application of a certain nonlinear mathematical formula. The tube is formed from plastic and has an internal diameter or radius which is partially blocked by a strut which obstructs the center of the air passage. Such devices are classified as a fixed orifice air flow sensor because the internal geometry of the device is in fact fixed. Current fixed orifice air flow sensors including the ""633 device, however, present problems which arise from turbulence in the air flow through the sensor which causes a nonlinear response of pressure change versus air flow rate through the device. To account for the nonlinear response, the ""633 system includes additional hardware which gain stages the pressure readings. Current fixed orifice devices such as the ""633 device also add a relatively high amount of resistance to the airway, which adds work to the patient just to breathe.
Other known air flow measuring devices rely on a variable area obstruction of the patient""s respiratory air passageway. Such devices are also tubular members which measure the pressure differential of the air flow through the tube. An example of a variable area obstruction air flow meter is described in U.S. Pat. No. 5,038,621 (referred to hereafter as xe2x80x9cthe ""621 devicexe2x80x9d). The obstruction in the ""621 air flow meter is comprised of a flexible elastic membrane which extends into the flow stream. A portion of the membrane deflects as the air flows through the obstruction. Variable obstruction air flow sensors normally produce a more linear pressure change versus flow rate measurement than do the current fixed orifice-type sensors, but a variable obstruction sensor also adds a relatively high amount of resistance to the air flow. Variable obstruction air flow sensors are also considerably more expensive to manufacture than are the fixed orifice type. The thin membrane in particular is difficult to manufacture to consistently tight specifications, so there is a significant amount of variability from one part to the other. The ""621 device is also made from multiple components, which of course require assembly that naturally adds to the cost of the device.
To obtain an accurate pressure differential measurement, a great amount of resistance by the obstruction in the air flow sensor is desired. This must, however, be balanced with the fact that a high resistance adds work to the patient just to breathe, which is, of course, a reason to keep the amount of resistance low. The internal geometry of the air flow sensor should also be of a type which provides the most accurate measurement possible over a range of air flow rates. Ideally a sensor exhibiting a linear or nearly linear pressure changes versus flow rate curve through the full range of anticipated respiratory pressures and flow rates is desired.
A further feature of a respiratory measurement system is the connector that is used to attach the air flow sensor to the microprocessor based analyzer module. In known prior art systems the connector for connecting the air flow sensor has been normally comprised of a first molded receptacle which is releasably connectable to a second molded receptacle, i.e., matching male and female receptacles. A typical example of such a device is the modular constructed connector disclosed in U.S. Pat. No. 5,197,895 (referred to hereafter as xe2x80x9cthe ""895 devicexe2x80x9d). The ""895 device and other similarly designed male/female-type connectors are normally quite expensive to treat as throw away or disposable devices. An improved connector for connecting the air flow sensor to the analyzer module, especially one which provides all of the necessary functions required of a connector for a respiratory system of the type presented here yet reduces or eliminates the number of components and thus reduces cost, is therefore desired.
Another feature of a respiratory measurement system is a means for purging the system of condensation or other debris that may block the airways or block the lumen tubing which attaches the air flow sensor to the analyzer module. In a patient ventilator circuit, natural cooling of the respiratory gases causes condensation of water vapor in the air tubes. If left undrained or unattended, the moisture will pool and may clog the tubing connected to the air flow sensor. A ventilator works by periodically compressing a volume of air which of course increases the air pressure in the ventilator circuit in order to force air into the patient""s lungs. The air is then returned to atmospheric pressures which allows the patient to exhale. This continuously fluctuating pressure in the breathing passage causes condensation in the ventilator circuit to migrate into the lumen tubes that are used for measuring the pressure differential in the air flow sensor mentioned above. Of course, a blockage in the lumen tubes will cause errors in the air flow measurement. In most known prior art systems, the lumen tubes are periodically purged with a short burst of air to clear any condensation or other obstruction that may be blocking the airway. One disadvantage of this method is that no measurements can be taken during the purge. Additionally, the purges normally occur at timed intervals, e.g., five minutes. In the event that a blockage occurs only one minute into the period, the air flow measurements will be incorrect for the remainder of the period. It is also difficult at times to determine whether a blockage has actually occurred because in some instances the signal may appear like an actual breath. An improved method of purging the pressure measuring airway passages in a respiratory air flow sensor is therefore desired.
In a respiratory measurement system it is also desirable to periodically measure the content of particular respiratory gases. There are many types of gas analysis procedures, but one commonly used method is infrared spectroscopy. In infrared spectroscopy, a sample of the gas extracted from the patient""s respirations is passed through a gas chamber located between an IR emitter and an IR detector. Particular gases, such as carbon dioxide (CO2) or nitrous oxide (N2O) are known to absorb particular wavelengths of light. The presence and concentration level of such a gas can therefore be determined by measuring the amount of light at the selected wavelength that has been absorbed by the gas sample. Known IR analyzers are prone to consuming more energy than is really necessary and inaccuracies due to unsatisfactory arrangement of the emitter and detector. An improved IR gas analyzer that better utilizes and focuses the infrared light energy is therefore desired. Known IR analyzers are also prone to failure after a short period of use, and so a compactly designed field replaceable IR analyzer module is also desired.
A respiratory measurement system is presented. The system is comprised of a respiratory air flow sensor, a microprocessor based analyzer module, a set of lumen tubes for connecting the air flow sensor to the module, a connector for connecting the lumen tubing to the module, an electronic subassembly for calculating the respiratory air flow rate based on pressure differential measurements received from the air flow sensor, an infrared emitter and detector subassembly for analyzing the gaseous content of specified respiratory gases of a patient, and a pneumatic subassembly for purging the system with a continuous low pressure and low volume air flow.
The system includes three airway passages in communication with the patient""s ventilated respiratory airway that is being monitored. Two of the airway passages are used to measure two airway pressures values for computing certain respiratory parameters. These parameters include inspired and expired respiratory air flow, pressure and volume. The third airway passage is used for extracting samples of the patient""s exhalations for analysis.
The air flow sensor is designed for placement into the ventilator circuit between the patient""s endotracheal tube and the ventilator. The airflow sensor is comprised of a tubular member providing a passage for the flow of air between the patient""s airway and the ventilator. The air flow sensor is a fixed orifice type sensor having a specified internal geometry which creates a resistance to the patient""s respirations.
The internal geometry of the air flow sensor is defined by the cylindrical walls of the tubular housing, and by a plurality of elongated aerodynamically curved vanes which extend from the internal surface to the cylindrical walls of the tubular member inwardly into the air passage. The internal geometry of the vanes creates a measurable pressure drop as air flows through the sensor. The unique internal geometry for a flow sensor as presented herein produces a resistance to the air flow which is far less turbulent than in current known fixed orifice sensors. The laminarization of the flow produces a nearly linear pressure versus flow rate response curve which greatly aids in the calculation of the patient""s respiratory air flow rates. In the present invention, the air flow rate is calibrated by measuring the pressure differential between two longitudinally spaced apart pressure measuring air ports within the air flow sensor. The sensor also includes a third air port for extracting a gas sample from the air flow for analysis.
The system further includes lumen tubing for connecting the air flow sensor to the analyzer module, and a novel connector for connecting the lumen tubing to the module. The lumen tubing is comprised of three tubes, two for transmitting the pressure signals and the third for transmitting the gas sample from the air flow sensor to the analyzer module. One of the tubes has an outer diameter which differs from the outer diameters of the other two tubes to ensure that the air flow sensor and analyzer module are connected together with the proper polarity with respect to the pressure changes being monitored.
The novel connector presented herein is comprised of a single molded housing specially designed to directly receive the triple lumen tubing. The tubing requires no special machining or preparation, the connector is instead capable of accepting just the raw square cut ends of the tubing, and accepts the tubing only if it is attached to the connector at the proper polarity. The matching male/female receptacles commonly found in the prior art are therefore unnecessary. The connector also includes a means for determining a positive connection of the lumen tubing to the connector, and a means for identifying the particular type of air flow sensor being used. In this regard the connector is comprised of two pairs of internal LED emitters and detectors strategically located around the periphery of the connector housing. The emitters and detectors are oriented such that when the clear plastic tubing is correctly placed into a socket in the connector housing, light energy from the emitters is refracted by the tubing and thereby directed to impinge onto the detector. The first emitter and detector combination is used to determine the presence of the lumen tubing. When the tubing is not present or not properly seated, the first detector remains dark and the analyzer inactive. The second emitter and detector combination is used for identification of the particular air flow sensing device in use.
The system further includes a pneumatic subassembly for purging the tubes and air sensor. The system places a very low, continuous flow of gas into the lumen tubing. A low flow continuous purge reduces the depth that compressed gas from the ventilator enters into the lumen tubing, and also inhibits water droplets from forming near the air port openings for the lumen tubing in the air flow sensor. A continuous purge therefore prevents obstructions from forming in the lumen tubing in the first instance.
The two lumen tubes used for measuring the pressure differential in the air flow sensor are both subjected to substantially the same continuous low pressure air flow. Because the air flows being introduced by the two lines being purged are of the same pressure, the two added pressure values effectively offset each other so that the measured pressure difference in the ventilator circuit being monitored remains unaffected. This approach also permits continuous monitoring without the interruptions encountered in the periodic purge systems of the prior art. A continuous purge also employs less hardware, eliminates certain valves necessarily required in a periodic purge system and therefore significantly reduces the cost of the system.
The system further includes a subassembly for analyzing certain respiratory gases through infrared spectroscopy. Molecules of diatomic gases, such as carbon dioxide (CO2) and nitrous oxide (N2O) absorb a specific wavelength of light energy. If one passes CO2 or N2O between an infrared emitter and an infrared detector, the amount of energy detected by the detector corresponds directly to the concentration of the gas of choice. The present invention includes a low power infrared CO2 or N2O sensor that has a small physical size, has a very short warm-up time, efficiently dissipates heat generated by the light source, and therefore optimizes the battery life. The IR sensor further includes a gas sample chamber which effectively utilizes an elliptical reflecting surface for the infrared light source. The light source and detector are both oriented longitudinally along the axis of the ellipse and the light source and detector are each located at the two focal points of the ellipse in order to optimize the light absorption reading.
The gas content analyzer additionally incorporates various compensatory features, allowing for automatic temperature and pressure compensation. For ease of use and serviceability, the present invention includes a field replaceable IR sensor subassembly which includes electronic memory chips for calibrating temperature and pressure constants used in respiratory measurements.
Accordingly, the primary objects of the present invention are to provide a respiratory measurement system which overcomes the problems of the prior art by providing an air flow sensor device having an internal geometry which optimizes the process for measuring the air flow rate and gas content of a patients respirations; to provide a novel connector for connecting lumen tubing from the air flow sensor directly to a gas analyzer module; to provide within the module an electronic subassembly for analyzing and calculating the air flow rate of respiratory gases; to provide in the analyzer module an infrared emitter and infrared detector subassembly for measuring the content of certain respiratory gases; to provide a means for continuously purging a respiratory measurement system with a low volume air flow; and to provide a system which includes disposable or replaceable components which are highly cost effective in their design and manufacture.
Other objects and advantages of the invention will become apparent from the following description which sets forth, by way of illustration and example, certain preferred embodiments of the invention.